Quantum-well Structures Research Articles

Constructing of amorphous/PbSe/amorphous multiple quantum wells with record high thermoelectric properties

Quantum well (QW) is one of the proposals to improve the thermoelectric properties. However, previous studies have shown that the large electron and phonon tunneling across the layers in semiconductor-based QWs and oxide-based QWs, respectively, are the two main challenges that need to be addressed. In this work, PbSe/amorphous-SrTiO3 (α-STO) supperlattices with a multiple quantum-well (MQW) structure can overcome both limitations. We demonstrate that the PbSe/α-STO MQW shows a high power factor of ∼ 24.5 μW cm−1K−2 at room temperature due to the strong quantum confine effect. In addition, the amorphous-SrTiO3 layer can engender intrinsically low thermal conductivity (0.27± 0.05 W m−1 K−1). The maximum estimated zT value is 2.7 for PbSe/α-STO MQW at room temperature, which is more than 1800 % higher than that of pure PbSe films (zT = 0.15).

First-principles investigation of the effects of excess carriers on the polytype stability and stacking fault energies of SiC

The characteristic polytype behaviors of SiC and accompanying low stacking fault energies are known to cause engineering issues, including polytype inclusions and bipolar degradation. The dependence of the relative stability of SiC polytypes and stacking fault energies on excess carrier concentration was investigated using first-principles calculations. The relative energy of 2H-, 4H-, and 6H-SiC to 3C-SiC increased with the excess electrons over 2 × 1019 cm−3, while the energy variation with excess holes was small. The stacking fault energies in 4H-SiC also exhibited a significant decrease with excess electrons over 1.0 × 1019 cm−3, whereas this change was minor with excess holes. These excess carrier dependencies were attributed to variations in the bandgap between polytypes. The energy level of the excess electrons was at the conduction band minimum; this was lowest in 3C-SiC, which had the lowest bandgap energy. Consequently, the energy of 3C-SiC with excess electrons was lower than that of other polytypes. Conversely, the valence band maximum lacked electrons when excess holes were present, resulting in a small difference among the Fermi levels of the polytypes. Hence, the energy difference between the SiC polytypes was similar for excess holes. Similarly, the stacking faults in SiC exhibited quantum-well structures by incorporating other polytypes with different bandgaps. With excess electrons, the Fermi level within the stacking faults was lower than that in the bulk crystals. Consequently, the stacking fault energy decreased for the same reason that the energy in 3C-SiC decreased under excess electron conditions.

Generation of an ultrahigh-repetition-rate optical half-cycle pulse train in the nested quantum wells.

We propose a simple quantum system, namely, a nested quantum-well structure, which is able to generate a train of half-cycle pulses of a few-femtosecond duration when driven by a static electric field. We theoretically investigate the emission of such a structure and its dependence on the parameters of the quantum wells. It is shown that the production of a regular output pulse train with tunable properties and the pulse repetition frequencies of tens of terahertz is possible in certain parameter ranges. We expect the suggested structure can be used as an ultra-compact source of subcycle pulses in the optical range.

GaAs/GaInNAs core-multishell nanowires with a triple quantum-well structure emitting in the telecommunication range

Semiconducting nanowires (NWs) fabricated from III–V materials have gained significant attention for their application in advanced optoelectronic devices. Here, the growth of GaAs/GaInNAs/GaAs core-multishell NWs with a triple quantum-well structure, having about 2% N and 20% In, is reported. The NWs are grown via selective area plasma-assisted molecular beam epitaxy on patterned Si(111) substrates with SiO2 mask holes. The nucleation and growth of the GaAs nanowires' core are carried out by Ga-induced vapor–liquid–solid growth at the open holes. Finely controlled, vertically aligned, regular core-multishell NWs with uniform wire length and diameter are obtained with a 96% yield and targeted nitrogen concentrations of 0%, 2%, and 3%. The GaInNAs NWs exhibit a spectral red shift relative to the GaAs NWs' peak. Their emission wavelength increases with the N content reaching up to 1.26 μm, which makes them a promising tool in telecommunication light sources.

Linewidth enhancement factor of type-II red InGaN/GaNSb/GaN quantum-well lasers

We investigate the linewidth enhancement factor of the type-II red InGaN/GaNSb/GaN quantum-well (QW) laser, employing a non-Markovian gain model with many-body effects included. It is shown that the linewidth enhancement factor of the type-II red InGaN/GaNSb/GaN QW structure is almost independent of the peak-gain coefficient. This behavior is contrasted with that of the conventional type-I InGaN/GaN QW structure, whose linewidth enhancement factor increases as the peak-gain coefficient increases. These results can be explained by the peak-gain dependencies of the differential refractive-index change and the differential gain. Moreover, the type-II red InGaN/GaNSb/GaN QW laser yields much smaller values of the linewidth enhancement factor than the conventional type-I InGaN/GaN QW laser. The type-II red InGaN/GaNSb/GaN QW laser with a relatively small, excitation-independent linewidth enhancement factor is expected to be highly useful for many practical applications.

Improvement in quantum efficiency of green GaN-based micro-LED by trapezoidal quantum well

The main challenges in the application of GaN-based full-color micro-light-emitting diodes (micro-LEDs) are size-dependent effects and low quantum efficiency, especially for green micro-LEDs. In this study, we propose to use a trapezoidal quantum well (QW) structure in GaN-based green micro-LEDs. The trapezoidal QW structure is able to improve the electron hole wave function overlap, thus increasing the external quantum efficiency (EQE). Green micro-LEDs with active region size of 75 × 64 μm2 are fabricated by metal organic chemical vapor deposition. The peak EQE of the green trapezoidal QW micro-LED reaches 15.8% at a low injection current density of 0.6A/cm2, which is 1.96 times higher than that of conventional rectangular QW. The test results indicate that the trapezoidal QW structure greatly improves the quantum efficiency of green micro-LED, inhibits the blue shift of peak wavelength and improves color purity, which paves the way to single substrate full-color micro-LED display with high-performance. The technology proposed in this paper does not change the production line equipment, but improves the traditional process, so it is expected to significantly improve the external quantum efficiency of the existing green micro-LED under the existing equipment. The production technology of existing InGaN QW micro-LED is directly adopted, and the EQE can be further improved if some new dry etching and passivation methods are applied.

Diversity Analysis of Defect Distribution and Carriers Quantization in Cu 2 ZnSnS 4/Cu2ZnSn(S,Se) 4 Kesterite Quantum Well Solar Cell

Kesterite materials, with a major advantage of direct bandgap and earth abundant material,are popular for low cost and thin film solar cell applications. Despite the popularity, the material fails to achieve significant efficiency due to the emergence of defect states during the growth process. In this paper multiple approaches have been investigated focusing on implementation of Quantum Wells(QW) to enhance the performance of solar cell. QW structure are created using CZTS x Se 1−x and Cu 2 ZnSnS 4 as QW and barrier material layers respectively. The performance evaluation of the QW solar cell is also carried out with the presence of most popular donor-like defect states in a gaussian and tail distribution way. The behavior of the device is investigated in presence of broad range of QWs from 2 to 100QWs to ensure the effect of defects on rise in QWs. The worst case performance of the solar cell is obtained to be efficiency of 9.6% under a high level of defect states. A remarkable observation on the effect of increase in number of QWs and defect states on solar cell parameters solar cell are concluded.

Surface plasmon coupling effects on the photon color conversion behaviors of colloidal quantum dots in a GaN nanoscale hole with a nearby quantum-well structure.

To improve color conversion performance for color display application, we study the near-field-induced nanoscale-cavity effects on the emission efficiency and Förster resonance energy transfer (FRET) under the condition of surface plasmon (SP) coupling by inserting colloidal quantum dots (QDs) and synthesized Ag nanoparticles (NPs) into surface nano-holes fabricated on a GaN template and an InGaN/GaN quantum-well (QW) template. In the QW template, the inserted Ag NPs are close to either QWs or QDs for producing three-body SP coupling to enhance color conversion. Time-resolved and continuous-wave photoluminescence (PL) behaviors of the QW- and QD-emitting lights are investigated. The comparison between the nano-hole samples and the reference samples of surface QD/Ag NP shows that the nanoscale-cavity effect of the nano-hole leads to the enhancements of QD emission, FRET between QDs, and FRET from QW into QD. The SP coupling induced by the inserted Ag NPs can enhance the QD emission and FRET from QW into QD. Its result is further enhanced through the nanoscale-cavity effect. The relative continuous-wave PL intensities among different color components also show the similar behaviors. By introducing SP coupling to a color conversion device with the FRET process in a nanoscale cavity structure, we can significantly improve the color conversion efficiency. Simulation results confirm the basic observations in experiment.

A2Bn-1PbnI3n+1 (A = BA, PEA; B = MA; n = 1, 2): Engineering Quantum-Well Crystals for High Mass Density and Fast Scintillators.

Quantum-well (QW) hybrid organic-inorganic perovskite (HOIP) crystals, e.g., A2PbX4 (A = BA, PEA; X = Br, I), demonstrated significant potentials as scintillating materials for wide energy radiation detection compared to their individual three-dimensional (3D) counterparts, e.g., BPbX3 (B = MA). Inserting 3D into QW structures resulted in new structures, namely A2BPb2X7 perovskite crystals, and they may have promising optical and scintillation properties toward higher mass density and fast timing scintillators. In this article, we investigate the crystal structure as well as optical and scintillation properties of iodide-based QW HOIP crystals, A2PbI4 and A2MAPb2I7. A2PbI4 crystals exhibit green and red emission with the fastest PL decay time <1 ns, while A2MAPb2I7 crystals exhibit a high mass density of >3.0 g/cm3 and tunable smaller bandgaps <2.1 eV resulting from quantum and dielectric confinement. We observe that A2PbI4 and PEA2MAPb2I7 show emission under X- and γ-ray excitations. We further observe that some QW HOIP iodide scintillators exhibit shorter radiation absorption lengths (∼3 cm at 511 keV) and faster scintillation decay time components (∼0.5 ns) compared to those of QW HOIP bromide scintillators. Finally, we investigate the light yields of iodide-based QW HOIP crystals at 10 K (∼10 photons/keV), while at room temperature they still show pulse height spectra with light yields between 1 and 2 photons/keV, which is still >5 times lower than those for bromides. The lower light yields can be the drawbacks of iodide-based QW HOIP scintillators, but the promising high mass density and decay time results of our study can provide the right pathway for further improvements toward fast-timing applications.

980 nm electrically pumped continuous lasing of QW lasers grown on silicon.

Investigation of high-performance lasers monolithically grown on silicon (Si) could promote the development of silicon photonics in regimes other than the 1.3 -1.5 µm band. 980 nm laser, a widely used pumping source for erbium-doped fiber amplifier (EDFA) in the optical fiber communication system, can be used as a demonstration for shorter wavelength lasers. Here, we report continuous wave (CW) lasing of 980 nm electrically pumped quantum well (QW) lasers directly grown on Si by metalorganic chemical vapor deposition (MOCVD). Utilizing the strain compensated InGaAs/GaAs/GaAsP QW structure as the active medium, the lowest threshold current obtained from the lasers on Si was 40 mA, and the highest total output power was near 100 mW. A statistical comparison of lasers grown on native GaAs and Si substrates was conducted and it reveals a somewhat higher threshold for devices on Si. Internal parameters, including modal gain and optical loss are extracted from experimental results and the variation on different substrates could provide a direction to further laser optimization through further improvement of the GaAs/Si templates and QW design. These results demonstrate a promising step towards optoelectronic integration of QW lasers on Si.

Design of InGaN-ZnSnGa2N4 quantum wells for high-efficiency amber light emitting diodes

A novel type-II InGaN-ZnSnGa2N4 quantum well (QW) structure is proposed based on recent experimental achievements for the successful epitaxy of ZnSnN2-GaN alloys and the determination of their band offsets with GaN. The simulation results indicate that this structure is promising as the active region for high-efficiency InGaN-based amber (λ ∼ 590 nm) light-emitting diodes (LEDs). The hole wavefunction in the valence band is better confined with the insertion of a monolayer scale of ZnSnGa2N4 into the InGaN QW while the electron wavefunction in the conduction band is better confined with the incorporation of an AlGaN layer in the GaN quantum barrier. The band structure of the InGaN-ZnSnGa2N4 QW is numerically simulated based on the experimentally measured band offsets between ZnSnGa2N4 and GaN. With the InGaN-ZnSnGa2N4 QW design, a low In content (20%) is required in the InGaN layer to reach a peak emission wavelength of ∼590 nm, yet an In composition of 25% is needed to reach the same emission wavelength for a conventional InGaN QW with the same layer thicknesses. Moreover, the electron-hole wavefunction overlap (Гe1−hh1) for the InGaN-ZnSnGa2N4 QW design reaches 18% for an emission wavelength at ∼590 nm. This result is much improved over the conventional InGaN QW overlap of 5% emitting at the same wavelength. The increase in electron-hole wavefunction overlap results in an approximately 14 times enhancement in the predicted spontaneous emission radiative recombination rate of the InGaN-ZnSnGa2N4 QW as compared to that of the conventional InGaN QW. This InGaN-ZnSnGa2N4 QW structure design can be promising to pave a new way to achieve high efficiency amber LEDs.

InGaN Laser Diodes-Gain, Spectra and Nonlinear Dynamics

In this era, short-wavelength laser diodes with quantum-well (QW) structures offer plenty of opportunities for improvement in laser performance and receive widespread attention. Therefore, we develop dynamic model of the violet-blue InGaN laser diodes (LDs) and discuss basic features of the device in this paper. We investigate longitudinal mode dynamics through detail numerical simulation of the rate equation model of the quantum-well LDs. We selectively observe several effects such as relaxation oscillation, mode competition, intensity noise etc. From the dynamic behavior of the gain spectrum and time-varying modal photon numbers we found that the higher intensity noise of the quantum-well structure was due to random fluctuation with time among the different modes. The results were explained considering the previously published findings to confirm the validity of the proposed rate equation model of the quantum-well structures. DUJASE Vol. 7 (2) 62-67, 2022 (July)

Simultaneous light emission and detection of an AlGaInP quantum well diode

When a quantum well (QW) diode is biased with a forward voltage and illuminated with an external shorter-wavelength light, the device simultaneously emits and detects light, with the injected current and the induced current mixed inside the wells. Separating these superimposed and dynamic electrical signals is useful for the development of multifunctional displays that can simultaneously transmit and receive light signals. By utilizing the unique overlap between the electroluminescence and detection spectra, we establish a wireless optical communication system using two AlGaInP diodes that have identical QW structures. The communication distance is 25 m, with one diode functioning as the transmitter and the other as the receiver. In particular, at the receiver end, the QW diode demonstrates simultaneous light emission and reception ability, and the mixed signals can be efficiently extracted, suggesting great potential for applications from light communication to advanced displays.

Enhanced Color Conversion of Quantum Dots Located in the Hot Spot of Surface Plasmon Coupling

For improving the performance of the photon color conversion from the energy of an InGaN/GaN quantum-well (QW) structure into the longer-wavelength emission of a colloidal quantum dot (QD), we insert the photoresist solution of the QD into a surface nano-hole, whose bottom face is about 10 nm higher than the top QW. The sample top surface is then cleaned for depositing Ag nanoparticles (NPs) to induce the surface plasmon (SP) coupling with the QW structure such that the QDs inside the nano-holes are located in the region of strong field distribution (hot spot). In this situation, the SP coupling can enhance the Förster resonance energy transfer (FRET) from QW into QD and the QD emission efficiency for producing a stronger color conversion process. The nanoscale-cavity effect of the nano-hole structure can further strengthen the SP-coupling enhanced FRET and QD emission.

Navigating the Site-Distinct Energy Conversion Properties of Perovskite Quantum Wells

Two-dimensional (2D) halide perovskites (HPs) are self-assembled multi-quantum wells with excitonic peculiarities. They afford favorable moisture and photothermal stability compared to traditional 3D counterparts. However, the energy conversion properties of 2D HPs are highly site-distinct, and they are found to have complicated origins that are associated with diverse local exciton relaxation dynamics, hence causing researchers severe bewilderment when targeting customized applications. This Review intends to offer a thorough survey on this topic. We first introduce the crystallographic structures of 2D HPs and how they render a quantum-well electronic structure. Subsequently, we elaborate on the exciton transport mechanisms in 2D HPs to interpret how excitons arrive at various local sites. Then we concretely introduce the diversiform local/bulk exciton relaxation dynamics that give rise to the wealthy energy conversion attributes. Finally, challenges and opportunities for better understanding and manipulating the exciton dynamics of this promising class of materials are discussed.

Double-Barrier Quantum-Well Structure: An Innovative Universal Approach for Passivation Contact for Heterojunction Solar Cells

The main drawbacks of modern solar-cell technologies are low-quality surface passivation, recombination losses, and carrier selectivity, which limit their efficiency. Therefore, this study proposes an innovative universal approach for a double-barrier two-dimensional (2D) quantum well (QW) passivation structure for solar cells. To this end, c-Si solar cells were examined as model cells. Preliminary investigations (e.g., contact resistance, passivation, and recombination current density) were conducted with a stack of SiOx/nc-Si/SiOx QW on n-type surfaces, and excellent results were obtained with a 30 nm-thick nc-SiOx(n) carrier-selective layer. Furthermore, the effects of different QW thicknesses and doping doses on the surface passivation of such contacts were studied, and the best results were achieved for a 5 nm QW. These QWs also exhibited a low degree of dopant diffusion, which was suppressed by the double SiOx layer. Furthermore, the 2D QW passivation structure with carrier-selective layers, which was denoted as a heterojunction with quantum well (HQW) solar cell, exhibited an excellent passivation improvement and had a lifetime (τeff) of 2746 μs and an implied open-circuit voltage (iVoc) of 736 mV for a 5 nm 2D QW structure. Moreover, a fabricated 5 nm 2D QW-based silicon heterojunction (SHJ) solar cell exhibited an open-circuit voltage (Voc) of 732.5 mV, a short-circuit current density (Jsc) of 39.5 mA/cm2, a fill factor (FF) of 77.95%, and an efficiency (Eff) of 22.55%. To validate these findings, theoretical calculations were performed using the experimental results, which confirmed the resonance tunneling of charge carriers across the 5 nm HWQ structure.

Dynamics of 2nd quantized state laser oscillation in gain-switched quantum-well semiconductor lasers

The dynamics of second-quantized-state laser oscillation were investigated for semiconductor laser diodes with quantum-well structures inside. We found that the second-quantized state often dominates laser oscillation instead of the first-quantized state under intensive pulse excitation, while the DC bias superposition tends to suppress the second-quantized-state oscillation. The operation characteristics were studied in detail through experimental studies and numerical calculations.

Effects of Surface Plasmon Coupling on the Color Conversion of an InGaN/GaN Quantum-Well Structure into Colloidal Quantum Dots Inserted into a Nearby Porous Structure

To further enhance the color conversion from a quantum-well (QW) structure into a color-converting colloidal quantum dot (QD) through Förster resonance energy transfer (FRET), we designed and implemented a device structure with QDs inserted into a GaN nano-porous structure near the QWs to gain the advantageous nanoscale-cavity effect. Additionally, surface Ag nanoparticles were deposited for inducing surface plasmon (SP) coupling with the QW structure. Based on the measurements of time-resolved and continuous-wave photoluminescence spectroscopies, the FRET efficiency from QW into QD is enhanced through the SP coupling. In particular, performance in the polarization perpendicular to the essentially extended direction of the fabricated pores in the nano-porous structure is more strongly enhanced when compared with the other linear polarization. A numerical simulation study was undertaken, and showed consistent results with the experimental observations.

Surface plasmon coupling effects on the behaviors of radiative and non-radiative recombination in an InGaN/GaN quantum well

Although surface plasmon (SP) coupling has been widely used for enhancing the emission efficiency of an InGaN/GaN quantum well (QW) structure, the interplay of the carrier transport behavior in the QW with SP coupling, which is a crucial mechanism controlling the SP-coupling induced QW emission enhancement, is still an issue not well explored yet. To understand the effects of SP coupling on the radiative and non-radiative recombination behaviors of carriers in a QW structure, the temperature-dependent time-resolved photoluminescence spectroscopies of two QW samples of different indium contents with surface Ag nanoparticles are studied. A two-single-exponential model is used for calibrating their radiative and non-radiative decay times. The SP coupling process, which transfers carrier energy from a QW into the SP resonance mode for effective radiation and increases the effective radiative recombination rate, produces energy-dependent carrier depletion and, hence, disturbs the quasi-equilibrium condition of carrier distribution. In this situation, a strong carrier transport process occurs targeting a new quasi-equilibrium condition that enhances non-radiative recombination and, hence, reduces the benefit of using the SP coupling technique. To alleviate this problem of SP-coupling induced energy loss, a weak energy-dependent or broad-spectrum SP coupling process is recommended.

Cesium doping for improving performance of inverse-graded 2D (CMA)&lt;sub&gt;2&lt;/sub&gt;MA&lt;sub&gt;8&lt;/sub&gt;Pb&lt;sub&gt;9&lt;/sub&gt;I&lt;sub&gt;28&lt;/sub&gt; perovskite film and solar cells

Compared with three-dimensional (3D) perovskites, two-dimensional (2D) perovskites have excellent environmental stability. However, the efficiency of 2D perovskite solar cells is still lower than that of their 3D counterparts owing to the poor carrier transport. In order to improve the efficiency of 2D perovskite solar cells, the cesium (Cs) doping 2D (CMA)&lt;sub&gt;2&lt;/sub&gt;MA&lt;sub&gt;8&lt;/sub&gt;Pb&lt;sub&gt;9&lt;/sub&gt;I&lt;sub&gt;28&lt;/sub&gt; films with an inverse gradient structure (small &lt;i&gt;n&lt;/i&gt; quantum well (QW) located at the surface and large &lt;i&gt;n&lt;/i&gt; QW at the bottom) are prepared. The inverse gradient QW structure has the advantages of being more moisture-resistant (small &lt;i&gt;n&lt;/i&gt; QW protects the vulnerable large &lt;i&gt;n&lt;/i&gt; QW from being attacked by water molecules) and favoring self-driven charge transport (type-II band alignment between different phases). The results show that the quality and thermal stability of 2D (CMA)&lt;sub&gt;2&lt;/sub&gt;MA&lt;sub&gt;8&lt;/sub&gt;Pb&lt;sub&gt;9&lt;/sub&gt;I&lt;sub&gt;28&lt;/sub&gt; film can be effectively improved by using Cs doping. The SEM images show that the film grains become larger, and the surface is smoother and more compact with CsI content increasing. When CsI increases to 15%, the surface becomes coarse. From the XRD, it can be seen that the crystallinity of perovskite is improved with CsI doping, and it reaches saturation when CsI content increases to 10%. The PL intensity of the film with 5% CsI is higher than the others’, implying a relatively low non-radiative recombination loss and low defect state in that film. Therefore, the minority carrier average lifetime of film doped with 5% CsI is the longest. The absorption is almost unchanged when CsI is doped. The thermal stability of film can be effectively improved when CsI exceeds 10%. Considering the SEM images, crystallinity, PL intensity, light absorption and thermal stability of perovskite, the optimized CsI doped concentration is 10% in our work. Finally, the highest efficiency of (CMA)&lt;sub&gt;2&lt;/sub&gt;MA&lt;sub&gt;8&lt;/sub&gt;Pb&lt;sub&gt;9&lt;/sub&gt;I&lt;sub&gt;28&lt;/sub&gt; perovskite solar cells doped with 10% CsI content reaches to 14.67%, with a short-circuit current density (&lt;i&gt;J&lt;/i&gt;&lt;sub&gt;SC&lt;/sub&gt;) of 23.16 mA/cm&lt;sup&gt;2&lt;/sup&gt;, an open-circuit voltage (&lt;i&gt;V&lt;/i&gt;&lt;sub&gt;oc&lt;/sub&gt;) of 1.05 V and a fill factor (FF) of 60.75%. The efficiency of the undoped cells is 10.06%, which is lower than that of CsI doped cells (10%). Therefore, CsI doping is an effective method to further improve the efficiency and thermal stability of 2D perovskite solar cells.